Most ion-exchange resins are produced by chemically attaching ion-exchange functional groups to crosslinked polymer beads. The utility of an ion-exchange resin depends to a large extent on the physical properties of the polymer beads used, such as the uniformity of bead size and the extent of crosslinking. The extent of crosslinking in a polymer bead can strongly affect the pore size, crush strength, and susceptibility to oxidation of the resin produced from that bead. Susceptibility to oxidation is generally a problem in cation-exchange resins, but not in anion-exchange resins. The functional groups in anion-exchange resins are more susceptible to oxidation than the polymer molecules which form the structural framework of the resins. Thus, the functional groups effectively protect the polymer backbones of anion-exchange resins from oxidative attack. The functional groups of cation-exchange resins, however, offer no such protection. Consequently, cation-exchange resins are particularly susceptible to oxidative cleaving of the polymer backbone and to leaching of the degradative byproducts of that oxidation. For further discussion of oxidation and leaching in cation-exchange resins, see J. R. Stahlbush et.al., "Prediction and Identification of Leachables from Cation-Exchange Resins," Proceedings of the 48th International Water Conference, 67-77, 1987. Some methods for improving the oxidative stability of cation-exchange resins are described in the literature, e.g., U.S. Pat. No. 4,973,607 in which cation-exchange resins are treated with an antioxidant or U.S. Pat. No. 5,302,623 in which an oxidation-stabilizing moiety is incorporated into the styrenic copolymer backbone.
Suspension polymerization is the traditional method used to manufacture polymer beads for ion-exchange resins. In suspension polymerization, monomers are introduced into a reactor containing an aqueous solution, where they are formed into droplets by mechanical agitation. The monomers are then polymerized to form polymer beads. The size and size distribution of the beads produced in this manner depend on a variety of factors, including the rate of agitation, the type of suspending agent, and the amount of monomer used compared to the amount of aqueous solution used. For a good general discussion of suspension polymerization, see H. F. Mark et.al., Encyclopedia of Polymer Science and Engineering, 2nd ed., John Wiley & Sons, 16, 443-471 (1989).
Many methods for crosslinking the polymer beads produced by suspension polymerization are known. The literature describes methods for crosslinking such beads during polymerization, after polymerization or both. Crosslinking is accomplished during polymerization by using a mixture of monomers which includes a crosslinker such as a polyvinyl aromatic monomer. Mixtures of styrene and divinylbenzene are commonly used for this purpose; see, for example, U.S. Pat. No. 3,792,029. The literature also describes methods for increasing the amount of crosslinking in a lightly crosslinked copolymer bead by post-crosslinking after the initial copolymerization. Alkylene bridge formation is one such method which has been employed to post-crosslink lightly crosslinked copolymer beads; see, for example, U.S. Pat. Nos. 4,263,407 and 4,950,332. Finally, the literature describes methods for crosslinking beads of linear polymer chains, such as those produced by suspension polymerization of styrene. U.S. Pat. No. 4,177,331 describes a process for sulfone-crosslinking linear polystyrene beads. In summary, a wide variety of methods are known for obtaining crosslinked polymer beads suitable for functionalization to produce cation- or anion-exchange resins.
However, polymer beads produced by conventional suspension polymerization are not ideal raw materials for ion-exchange resin production, due to a general lack of uniformity in bead size. This lack of size uniformity can adversely affect the performance of an ion-exchange resin produced from such beads. U.S. Pat. No. 5,081,160 discusses the effect of non-uniformity in bead size on ion-exchange resin performance. In general, smaller resin beads have shorter diffusion paths resulting in improved exchange kinetics when compared to larger resin beads. But, smaller beads also tend to increase the pressure drop across a resin bed limiting the amount of liquids that can be processed. The availability of resin beads of fairly uniform size allows for the use of generally smaller beads with their desirable exchange kinetics without otherwise contributing toward unacceptably high pressure drops. Uniform-size ion-exchange resins, therefore, tend to have superior properties in comparison to resins with non-uniform size distributions.
Greater uniformity in bead size can be economically obtained by using a seeded process for polymer bead formation, rather than a traditional batch suspension polymerization process. In a seeded process, one starts with lightly crosslinked or uncrosslinked polymer seed beads of relatively uniform size produced by a suspension polymerization process. The seed beads are then imbibed with a monomer mixture containing both vinyl aromatic monomer and polyvinyl aromatic monomer, and the imbibed monomers polymerized to form seeded copolymer beads; see, for example, U.S. Pat. No. 4,419,245. Additional monomer mixture can also be added to the seeded copolymer beads intermittently or continuously during the polymerization; see, for example, U.S. Pat. No. 4,564,644.
Seeded copolymer beads produced by any one of the methods described above can be functionalized to produce either cation- or anion-exchange resins. However, such beads have been found to be of limited utility in cation-exchange resin production because of their high susceptibility to oxidation and leaching. Cation-exchange resins today, therefore, continue to be produced primarily from polymer beads manufactured by traditional suspension polymerization rather than by seeded methods. However, while functionalized polymer beads produced by suspension polymerization do not generally exhibit the same problems with oxidation and leaching exhibited by functionalized seeded beads, they do tend to lack size uniformity. What is needed is a method of modifying seeded polymer beads in such a way that cation-exchange resins made from those beads have a high amount of resistance to oxidation and leaching.
Some sulfone bridges are known to form between the polymer molecules of a polystyrene bead during the standard sulfonation process used to functionalize a polymer bead to convert it into a cation-exchange resin. Sulfone crosslinks were found to form during sulfonation of polystyrene beads when either oleum, sulfur trioxide or chlorosulfonic acid are used in the sulfonation reaction; see, for example, A. S. Goldstein, "Sulfone Formation During Sulfonation of Crosslinked Polystyrene," Ion Exchange & Membranes, 1, 63-66 (1972). However, the number of sulfone crosslinks formed between polymer molecules is minimal in a standard sulfonation process. In fact, the literature recommends choosing sulfonation conditions which minimize the formation of sulfone bridges when functionalizing polymer beads; see, for example, Goldstein, supra, or D. H. Freeman et.al., "Homogeneous Sulfonation of Styrene-Divinylbenzene Copolymers with Oleum in Organic Solvents," IsraelJournal of Chemistry, 7, 741-749 (1969). Sulfone bridges are portrayed as the products of an undesirable side reaction, a side reaction which reduces the number of sulfonate or sulfonic acid functional groups available for ion-exchange.
Copolymers of styrene and vinylbenzocyclobutene are known to form crosslinks with one another when thermally activated; see, for example, U.S. Pat. No. 4,698,394.
Lastly, in U.S. Pat. Nos. 4,263,407 and 4,950,332, polymeric adsorbents exhibiting improved porosity and adsorption characteristics are produced from lightly-crosslinked macroreticular aromatic copolymer beads by post-crosslinking the copolymer beads while in a swollen state. The post-crosslinking is accomplished by reacting the polymer with polyfunctional alkylating agents, polyfunctional acylating agents or sulfur halides in the presence of a Friedel-Crafts catalyst in either a single or consecutive reactions.